1914] Grhmell: Mammals and Birds of flic Colorado Valhy 2-17 



lish the point, trapped on dry surfaces, indicating a preference for 

 dryness. On thoroughly dried-out, rather high sand banks along the 

 river where the bottom strip was narrow, sign was noted to within a 

 few feet of the edge of the swift current. Individuals were trapped 

 on sand bars, at the time high and dry, but so slightly elevated that 

 rising water would at fir.st form islands of them and then engulf them 

 completely. 



No doTibt. as with Pcromysciis, aggre.ssive individuals of Perog- 

 nathiis 2)e)iicillohis are often thus caught and set adrift, thas giving 

 a chance for transfer from one side of the stream to tlie other. As 

 discussed in a general way elsewhere, there seems to he significant cor- 

 relation between the fact of this free invasion of tlie river bottom 

 and existence and similarity of the representatives of the species on 

 both sides of the river. 



The breeding season of jieniciUatiis is of greater duration than that 

 of the other pocket mice of the region, as shown by the following data. 

 A female taken April 19 contained five embryos ; another taken "Sidy 4 

 contained four embryos. Graj'-coated young-of-the-year were taken as 

 early as ^lareh 5 and at intervals from then on through Api-il and 

 into ]\Iay. 



After study of the Colorado River series in connection with the 

 120 additional specimens in the Museum from various localities in 

 the Salton Sink region of the Colorado desert, I have come to the 

 conclusion that the recognition of the name aiigustirostris as apply- 

 ing to an.v of the Colorado River specimens is impracticable. Osgood 

 (1900. pp. 45-47) described a race angustirostris from the Colorado 

 de.sert, type locality Carriso Creek, west side of the desert. The 

 Museum has a good topotype series, and also a series from Jlecea, in 

 the bed of the Salton Sink at the northwest end of Salton Sea. 



The characters a.ssigned to "aiigustirosfris'' were, as compared 

 with iwiiiciUatas, small size, less massive skidl, and longer and more 

 slender rostrum. Osgood referred his few specimens from the Colorado 

 River, from Ehrenberg and above, to penicillatus, while those from 

 Fort Yuma and all localities to the west he listed under angustirostris. 



Close scrutiny of the extensive material now at hand shows great 

 in,stability in all the alleged characters. There is as notable fluctuation 

 in size from place to place up and down the river, as across the Colorado 

 desert. The topotypes of angustirostris include some skulls as massive 

 as manv from IMellen. Arizona. 



